x86/reboot: KVM: Disable SVM during reboot via virt/KVM reboot callback

Use the virt callback to disable SVM (and set GIF=1) during an emergency
instead of blindly attempting to disable SVM.  Like the VMX case, if a
hypervisor, i.e. KVM, isn't loaded/active, SVM can't be in use.

Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230721201859.2307736-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/include/asm/virtext.h

@@ -133,12 +133,4 @@ static inline void cpu_svm_disable(void)
 	}
 }
 
-/** Makes sure SVM is disabled, if it is supported on the CPU
- */
-static inline void cpu_emergency_svm_disable(void)
-{
-	if (cpu_has_svm(NULL))
-		cpu_svm_disable();
-}
-
 #endif /* _ASM_X86_VIRTEX_H */

arch/x86/kernel/reboot.c

@@ -826,9 +826,6 @@ void cpu_emergency_disable_virtualization(void)
 	if (callback)
 		callback();
 	rcu_read_unlock();
-
-	/* KVM_AMD doesn't yet utilize the common callback. */
-	cpu_emergency_svm_disable();
 }
 
 #if defined(CONFIG_SMP)

arch/x86/kvm/svm/svm.c

@@ -39,6 +39,7 @@
 #include <asm/spec-ctrl.h>
 #include <asm/cpu_device_id.h>
 #include <asm/traps.h>
+#include <asm/reboot.h>
 #include <asm/fpu/api.h>
 
 #include <asm/virtext.h>
@@ -563,6 +564,11 @@ void __svm_write_tsc_multiplier(u64 multiplier)
 	preempt_enable();
 }
 
+static void svm_emergency_disable(void)
+{
+	cpu_svm_disable();
+}
+
 static void svm_hardware_disable(void)
 {
 	/* Make sure we clean up behind us */
@@ -5209,6 +5215,13 @@ static struct kvm_x86_init_ops svm_init_ops __initdata = {
 	.pmu_ops = &amd_pmu_ops,
 };
 
+static void __svm_exit(void)
+{
+	kvm_x86_vendor_exit();
+
+	cpu_emergency_unregister_virt_callback(svm_emergency_disable);
+}
+
 static int __init svm_init(void)
 {
 	int r;
@@ -5222,6 +5235,8 @@ static int __init svm_init(void)
 	if (r)
 		return r;
 
+	cpu_emergency_register_virt_callback(svm_emergency_disable);
+
 	/*
 	 * Common KVM initialization _must_ come last, after this, /dev/kvm is
 	 * exposed to userspace!
@@ -5234,14 +5249,14 @@ static int __init svm_init(void)
 	return 0;
 
 err_kvm_init:
-	kvm_x86_vendor_exit();
+	__svm_exit();
 	return r;
 }
 
 static void __exit svm_exit(void)
 {
 	kvm_exit();
-	kvm_x86_vendor_exit();
+	__svm_exit();
 }
 
 module_init(svm_init)